The present invention relates to a printing apparatus including an erasing mechanism for erasing printed characters and symbols, and specifically relates to a character erasable printing apparatus capable of surely erasing ink adhering heavily on the right-side or left-side edge part of a printed character.
Generally, character erasable printing apparatuses such as the typewriter comprises a printing apparatus for printing characters and symbols by hammering desired types on a type-wheel by a print hammer and an erasing mechanism capable of erasing characters and symbols printed by the printing mechanism.
For erasing characters and symbols by the erasing mechanism, the erasing mechanism adopts the cover lap method which corrects by adhering correcting agent onto the printed character or the lift-off method which corrects by lifting off ink on the printed character, and in recent years the lift-off system has been generally employed. In the lift-off method, a printed character or symbol to be erased is erased by hammering by a print hammer driven mechanically or by a solenoid through an erase ribbon.
Conventionally, for example, an erasing apparatus for impact printer is described in U.S. Pat. No. 4,307,971.
In this erasing apparatus, assuming errors of the stop positions of a print head at the print position and the erase position, first, the print head is moved from the print position in the direction of escapement by an amount of movement within a range of 2-20% of the amount of character escapement, then a first erasing operation being performed, and subsequently, the print head is moved from the print position in the direction reverse to that of escapement by an amount of movement within a range of 2-20% of the amount of character escapement, then a second erasing operation is performed.
In the printing apparatus describe in U.S. Pat. No. 4,388,005, by means of utilizing minute play .DELTA.S caused by errors in stop position of a stepping motor for driving the carriage and backlash in the driving gear mechanism and wire tension, when the carriage is moved in the first direction and then the carriage is moved in the direction opposite to the first direction, the carriage can be stopped at different two positions displaced by an amount of 2.DELTA.S relatively, where printing of bold face character and/or erasing of printed character can be carried out effectively.
In the printing apparatus described in U.S. Pat. No. 4,708,505, in order to avoid ink transfer from the correction ribbon to the paper in erasing a printed character at two or more impressions, a prescribed length of correction ribbon is fed after at least one erasing impession. Thus second erasing and third erasing can be conducted with fresh correction ribbon.
In an erasing apparatus described in the Japanese Patent provisional publication No. 147376/1985, for example, as shown in FIG. 9(a), first, a first erasing operation is performed at the position agreeing with the print position of a printed character `1`, and subsequently, as shown in FIG. 9(b), a print head is moved in the direction of spacing by a predetermined minute distance b, and then a second erasing operation is performed, and further as shown in FIG. 9(c), the print head is moved from the print position in the direction of backspacing by the predetermined minute distance b, and then a third or a third and a fourth erasing operation is performed.
In addition, numeral 29 designates a correction ribbon, numeral 34 designates ink of the printed character, numeral 34a designates ink lifted off from the printed character, and symbol P designates print paper
In the mechanical print hammer, to facilitate transfer of ink of a print ribbon onto print paper, as shown in FIG. 4 included in an embodiment of the present invention, on a head 7b of a print hammer, a large hammer weight 33 is mounted on the left side thereof, and a small hammer weight 32 is mounted on the right side thereof respectively, and when a type 5 is hammered by the print hammer, a turning torque is applied to the print hammer in the direction as shown by an arrow C along with a hammering force, and the type 5 sideslips in the direction as shown by an arrow D by a minute distance. As a result, as shown in FIG. 5, ink adheres to the right-side edge part of the character `I` in a particularly firm manner, and this edge part becomes a portion where ink is difficult to be erased.
However, in the conventional erasing apparatus as described in FIG. 9, the first erase position is made to agree with the print position (refer to FIG. 9(a)), and therefore the ink 34a having a width equal to that of the printed character adheres to the correction ribbon 29 (refer to FIG. 9(b)), and the second erasure is executed in this state, and therefore the ink 34a has already adhered to the portion of the correction ribbon 29 corresponding to the right-side edge part, and even when the third erasure is executed thereafter as shown in FIG. 9(c), the ink 34 is left without being stripped off certainly as shown in FIG. 9(d), thereby raising a problem.
Furthermore, in the erasing apparatus as described in the above-described Japanese Patent Publication No. 6992/1981, the edge part of the character to be erased whereto ink adheres heavily is not taken into account, and therefore a problem exists that ink of the edge part remains.